This invention relates generally to a differential gear assembly of a motor vehicle, and particularly to an assembly in which a crown wheel and a differential casing are welded together to form a unitary assembly.
As is well known, a differential gear assembly of a motor vehicle includes a differential casing in which planetary and output gears rotate in mesh to permit opposite wheels of an axle to turn at different speeds. The differential casing is mounted for rotation in an axle housing and is driven by a crown wheel fixed on the differential casing and itself driven by a vehicle engine.
Previously, it has been common for the annular crown wheel to be bolted to the differential casing. One reason for this construction is to ensure that an input ratio of a differential can be selected during manufacture. Another reason is to ensure that the crown wheel can be replaced as a service item. Additionally, the cost of a one piece component is very high.
Permanent welding of the crown wheel to the differential casing can reduce costs if the advantages of selection and serviceability are not required. Other advantages are potential weight reduction and a more compact design. However, the crown wheel is case hardened after final machining, whereas the differential casing is of e.g., forged steel. Although welding of case hardened steel to relatively soft forged steel is possible using a filler material such as nickel, the weld quality is difficult to control, and the welding process is expensive. As an alternative, it has been proposed to hard turn or grind through the case hardening of the crown wheel to the relatively soft core to expose material suitable for welding, but this is also time consuming and expensive.
It has also been proposed to machine a mounting face of the crown wheel as an outward stepped form before hardening to ease removal of the hard case. However, this is also disadvantageous because, typically, the crown wheel must be press-quenched during the hardening process to prevent warping thereof, and the stepped form requires centering of a press tool and a mounting fixture, which is time consuming and difficult to achieve with reliability. This problem is exacerbated because the crown wheel is immersed in oil during quenching, and thus difficult, if not impossible, to observe. This latter problem is also particularly troublesome in relation to truck axles, where numerous final drive ratios may be provided for use with a common differential gear assembly. Thus, for example, twelve different ratios (meaning twelve different crown wheels) are provided, each of which requires a respective mounting fixture during press-quenching. If the wrong fixture is used, at least the crown wheel will be made unusable, and possibly the fixture and/or press tool will be damaged.
DE-A-10238236 discloses a construction in which the crown wheel is welded to the differential housing by a radially applied weld. For this reason, a back face of the crown wheel is cut away to give access for a welding tool and is not flat. The same technique is used at a radial weld of the differential housing and a cover so that the differential housing is cut away (or relatively extended) in an axial direction.
What is required is a solution to the aforementioned difficulties to facilitate welding of the crown wheel and the differential casing.